Kepler repeated their experiments, and, striving as always to generalize his observations, he attempted to find the law that governed the observed change of direction which a ray of light assumes in passing from one medium to another.

Kepler measured the angle of refraction by means of a simple yet ingenious trough-like apparatus which enabled him to compare readily the direct and refracted rays.

He discovered that when a ray of light passes through a glass plate, if it strikes the farther surface of the glass at an angle greater than 45 degrees it will be totally refracted instead of passing through into the air.

He could not well fail to know that different mediums refract light differently, and that for the same medium the amount of light valies with the change in the angle of incidence.

He was not able, however, to generalize his observations as he desired, and to the last the law that governs refraction escaped him.